Gregorian telescope

A Gregorian telescope is an early type of reflecting telescope,
designed by the Scottish mathematician and astronomer James Gregory (1638–1675),
that uses two curved mirrors: a concave paraboloidal primary and a concave
ellipsoidal secondary. The primary collects and brings light to a focus,
while the secondary, positioned a little way beyond the primary's focal
plane, reflects the beam, diverging from the focus, back through a hole
in the center of the primary and out the bottom end of the instrument. Although
the Gregorian is free from chromatic and spherical aberration, it
requires a long telescope tube. It was rendered obsolete by the Cassegrain
telescope.